A) Field of the Invention
This invention relates to a manufacturing method of a semiconductor element.
B) Description of the Related Art
It has bee widely practiced that a GaN based light emitting diode is formed on a sapphire substrate. The sapphire substrate is insulative and has relatively low thermal conductivity; therefore, a GaN based light emitting diode from which a sapphire substrate is removed has been manufactured. Sapphire is transparent, and light in the ultraviolet region which a GaN based semiconductor absorbs passes through it. Therefore, a laser lift off (LLO) process for removing a sapphire substrate by using ultraviolet laser has been widely used.
A GaN based semiconductor layer is grown on a sapphire substrate, and a supporting substrate such as a Si substrate is bonded to a surface of the GaN based semiconductor layer via a bonding metal layer made of eutectic alloys or the like. Thereafter the sapphire substrate is removed by irradiating a laser from a side of the sapphire substrate. By the irradiation of the laser, the GaN based semiconductor is decomposed at an interface between the sapphire substrate and the GaN based semiconductor layer, and Ga metal and N2 gas are generated. By the generation of N2 gas, the sapphire substrate is removed. In case that the GaN based semiconductor layer is formed widely over the sapphire substrate, a pressure of the generated N2 gas may be high because it may be sealed locally. It has been well-known that the generation of high-pressure N2 gas may destruct a wide area of the GaN based semiconductor layer.
It has been suggested that the GaN based semiconductor layer grown on the sapphire substrate is divided into chip regions and removed from regions between chips in order to avoid the destruction of the GaN based semiconductor layer by the generation of the high-pressure N2 gas. For example, an n-type GaN based semiconductor layer, a GaN based semiconductor active layer and a p-type GaN based semiconductor layer are laminated on a sapphire substrate, masks each covering a chip region are formed, a street region where a surface of the sapphire substrate is exposed is formed by etching the GaN based semiconductor lamination, a supporting substrate such as a Si substrate or the like is bonded to the GaN based semiconductor lamination via a bonding metal layer made of eutectic alloys or the like, and thereafter the sapphire substrate is removed by irradiating a laser from a side of the sapphire substrate. By that, the sapphire substrate can be removed without destructing the semiconductor lamination.
In the above technique, the semiconductor lamination is removed from the street region and so the laser irradiates the bonding metal layer on the supporting substrate and laser ablation occurs. Metal vapored by the laser ablation deposits on a side of a chip, and it causes a short circuit and leakage (for example, refer to Japanese Laid-Open Patent No. 2007-134415).
Therefore, it has been suggested that chips are separated by forming a wide isolation trench (street) demarcating chip regions by etching a half of the thickness of the semiconductor lamination, depositing an insulating layer on side walls of the wide street region and forming a narrow isolation trench (street) piercing the semiconductor lamination to the surface of the substrate from a bottom surface of the wide street region by etching, and thereafter the sapphire substrate is removed by the laser lift off process.
According to the prior art, the periphery of each chip is etched until the growing substrate is exposed before the LLO process and so the laser is irradiated to the bonding metal layer (AuSn, etc.) on the supporting substrate at the LLO process. The light of 248 nm wavelength almost completely passes through the sapphire substrate; therefore, a region (street region) without the GaN layers is irradiated by the laser and so metal is sputtered from the bonding metal layer in the street region and adheres to side walls of the element, which may cause leakage and a short circuit.